Several methods are presently used to clean surfaces for the electronics industry. Solvent or chemical cleaning is used to remove contaminant films from surfaces. Since solvents are selected for the materials they can dissolve, an appropriate solvent must be chosen to remove contamination. Chemical solutions can be combined with megasonic or ultrasonic cleaners. These devices impart high energy sonic waves to the surface which can remove organic films, ionic impurities and particles as small as about 3,000 angstroms. However, solvent or chemical cleaning requires extremely pure and clean agents. High purity and cleanliness is difficult and/or expensive to achieve in liquid agents. In addition, the agent becomes progressively more contaminated as it is used and must be disposed of periodically. Failure to change the agent periodically causes redeposition of contaminants, which reduces the effectiveness of the cleaning process. Disposal of such agents frequently results in environmental damage. Also, such agents require special safety procedures during handling in order to minimize exposure to operators.
Gas jet and liquid spray cleaning are presently used to clean relatively large particles from silicon wafers. Gas jets, such as filtered nitrogen, are effective in removing particles smaller than about 50,000 angstroms. Smaller particles are more difficult to remove. This is because the adhesive force tending to hold the particle to the surface is approximately proportional to the particle diameter while the aerodynamic drag force by the gas tending to remove the particle is approximately proportional to the diameter squared. Therefore, the ratio of these forces tends to favor adhesion as the particle size shrinks. Also, smaller particles are not exposed to strong drag forces in the jet since they can lie within the surface boundary layer where the gas velocity is low. Liquid jets provide stronger shear forces to remove particles, but are expensive and/or difficult to obtain in high purity and may leave contaminating residues after drying. Also, a common liquid spray solvent comprising a chlorofluoro carbon, FREON TF, is environmentally damaging. Alternatively, the art has used exposure to ozone combined with ultraviolet light to decompose contaminating hydrocarbons from surfaces of semiconductors. However, this technique has not yet been shown to remove contaminating particles with any efficacy.
A recently developed cleaning technique involves the use of a carbon dioxide aerosol to sandblast contaminated surfaces. Pressurized carbon dioxide is expanded in a nozzle. The expansion drops the carbon dioxide pressure to atmospheric pressure. The resulting Joule-Thompson cooling forms solid carbon dioxide particles which traverse the surface boundary layer and strike the contaminated surface. In some cases the carbon dioxide forms a soft material which can flow over the surface, displacing particles without leaving a residue. The technique requires extremely clean and pure carbon dioxide. Trace molecular contaminants, such as hydrocarbons, in the feed gas can condense into solid or liquid particles on the surface. Carbon dioxide is difficult and/or expensive to provide in ultra high purity, such as with less than parts per million levels of trace purities. Because of this problem, the carbon dioxide cleaning technique has not yet been shown to be effective in ultra clean silicon wafer applications.
U.S. Pat. No. 3,545,996 discloses a device for placing a pattern on the surface of stainless steel by impact of a hard particulate treating material from a nozzle contained in a housing and having a shield 42 which focuses the impinging treating material. The patent is not directed to surface cleaning.
U.S. Pat. No. 4,084,357 discloses a valve cleaning chamber having a window 18 for viewing the cleaning operation. Nozzles within the housing of the apparatus are directed to valves and provide a jet of compressed air containing abrasive material for cleaning of the valve surfaces.
U.S. Pat. No. 4,631,250 discloses an apparatus for cleaning photo resist film from a semi-conductor using a spray of fine ice and carbon dioxide particles. The particular apparatus is further described in U.S. Pat. No. 4,747,421.
U.S. Pat. No. 4,793,103 discloses a cryogenic deflashing apparatus which uses cryogenic conditions to embrittle materials to be cleaned and deflashed and then blasts plastic pellets at the embrittled piece to be deflashed to remove the flashing components.
U.S. Pat. No. 4,817,652 discloses a cleaning apparatus which may use liquids and gases including argon. Liquid cleaning agents are used to float contaminants off the surface to be cleaned or the fluid may be frozen and then partially melted to remove contaminants locked in the frozen residual cleaning media.
U.S Pat. Nos. 4,832,753 and 4,936,922 disclose cleaning systems using droplets of solvent. The apparatus includes a tray which slides along gas bearing operated tubes.
U.S. Pat. No. 4,974,375 discloses a chamber for cleaning semi-conductor wafers with a jet of ice particles of ultra pure water. The spray of ice particles contacts the semiconductor wafer to be cleaned at an angle on a rotating surface driven by a motor.
U.S. Pat. No. 5,009,240 discloses an apparatus for cleaning semi-conductor wafers by spraying a blast of ice particles against the wafer in which residual ice is removed by evaporation.
U.S. Pat. No. 4,962,891 discloses a nozzle apparatus for dispensing a mixture of solid particles and gaseous carbon dioxide. This nozzle is used to clean small particles from a substrate.
U.S. Pat. No. 1,899,626 discloses a burner apparatus having a multi apertured nozzle configuration.
The present invention overcomes the drawbacks of the prior art by providing a controlled atmosphere apparatus which dispenses a controllably directed spray of sublimable frozen particles for cleaning using a precise tracking means to administer the cleaning in a careful and calibrated manner so as to overcome the deficiencies experienced by the prior art. These attributes are described with greater clarity with regard to the present invention which is set forth below.